Metal tantalum, being a valve metal, is able to form a dense oxidative film on a surface, so that the metal has the unilateral conduction property. Anode films prepared from the metal tantalum have stable chemical properties (particularly, the films are stable in an acidic electrolyte), a high electrical resistivity (7.5×1010 Ω·cm), a high dielectric constant (27.6), a small leakage current. In addition, the anode films further have the advantages of a broad working temperature range (from −80° C. to 200° C.), a high reliability, shock resistance, and a long life or the like. Thus, tantalum powder is an ideal material for preparing tantalum capacitors having a small volume and a high reliability. Since the tantalum capacitors have many advantages, they can be widely used in electronic devices of aviation, aerospace, communication, computer and cellphone.
The batch production of tantalum powder has a history 70-year or more. However, the production is developed in large scale and high speed merely in recent 50 years. As early as 1904, people employed carbon to reduce tantalum pentoxide, and the reduction product is refined at a high temperature and under vacuum, to obtain the first tantalum ingot having the ductility in the world. The fused salt electrolysis process which is developed afterward is economic, while requiring simple apparatus. Hence, the process is ever widely used. However, tantalum powder electrolyzed by conventional processes has simple particle shape, coarse particles, and a low specific capacitance, and thus the tantalum powder cannot meet the requirements of electronic industry to high capacitance tantalum powder. Carbothermal reduction of tantalum oxide, hydrogen reduction of tantalum pentoxide and aluminothermic reduction still are not applied in industrial productions. Tantalum powder prepared by the sodium reduction has the characteristics of a high purity, complex particle type, and a high specific capacitance, and thus the reduction process becomes a primary production process for tantalum powder at home and abroad. Now, manufacturers for tantalum powder in the world primarily include Cabot Group in U.S.A, HCST Group in German and Ningxia Orient Tantalum Industry Co. LTD. in China.
In view of material classification, tantalum powder special for the manufacture of a capacitor is called as a capacitor grade tantalum powder. Tantalum powder, according to used voltages, may be classified into high voltage tantalum powder (working voltage is higher than 35 V), intermediate voltage tantalum powder (working voltage is from 20 to 35 V), and low voltage tantalum powder (working voltage is below 20 V). Tantalum powder which are used in capacitors is about 60 to 70% of the total consumption of tantalum powder in the world. In particular, recently, with the rapid development of computer and electronic industry, tantalum powder is always required in a stable and raised trend, and it can be predicted that in the coming five to ten years, tantalum industry in the world will be continuously developed in a speed that the production amount is increased by 15% per year.
High voltage tantalum powder used in military products mainly is meant to 63V series tantalum powder manufactured by electron beam smelting, hydrogenation and pulverization, and its working voltage range can be expanded to between 50 V and 75 V. The high voltage tantalum powder is important in the fields of aviation, aerospace and military. Still some high voltage tantalum powder having the working voltage of from 35 to 50 V mainly includes intermediate voltage flaked tantalum powder and other tantalum powder having a high voltage resistance. Recently, intermediate voltage tantalum powder having the working voltage from 20 to 35 V is a hot spot of the later development in the art.
U.S. Pat. No. 3,647,415 discloses tantalum powder for electrolytic capacitors wherein tantalum particles in said powder have an average flakiness of 2 to 60, at least 80 percent by weight of said powder consisting of particles having a shorter breadth of 3 to 250 μm and said powder being free from particles having a shorter breadth of above 500 μm, and methods of preparing the same.
U.S. Pat. No. 4,441,927 discloses an ingot-derived agglomerated tantalum powder composition comprising a select granular tantalum powder and including a critical proportion of a select flaked tantalum powder. Both are derived from the tantalum ingot.
U.S. Pat. No. 4,555,268 is a method invention, relating to flaked tantalum powder having improved workability. The mixed powder is an agglomerated tantalum powder mixture made by electron beam smelting. It contains flaked tantalum powder (20-40%) and particular tantalum powder (70%). Prior to the mixing, the mixed tantalum powder are subjected to heat treatment at temperatures range from about 1250° C. to about 1550° C. for a period ranging from about 5 to about 120 minutes, so as to improve the workablity. The tantalum powder prepared according to the method in the patent is suitable for intermediate working voltage (20 V to 35 V).
U.S. Pat. No. 4,740,238 is concerned to an ingot-derived unagglomerated tantalum powder composition includes a platelet tantalum powder having an average Fisher sub-sieve size (FSSS) of less than two micrometers, preferably in the range of 0.6 to 1.1 micrometers, a Scott density not greater than 30 g/in3, and a BET surface area of at least 0.7 m2/g. The tantalum powder is tantalum powder in the pure flaked form.
U.S. Pat. No. 4,940,490 relates to an improved flaked tantalum powder and process for making the flaked powder are disclosed. The powder is characterized by having a Scott density greater than about 18 g/in3 and preferably at least about 90% of the flaked particles having no dimension greater than about 55 micrometers. Agglomerates of the flaked tantalum powder, provide improved flowability, green strength and pressing characteristics compared to conventional flaked tantalum powders. The improved flaked tantalum powder can be made by preparing a large flaked tantalum and then reducing the flaked size until a Scott density greater than 18 g/in3 is achieved. The tantalum powder is a tantalum powder in the pure flaked form.
In addition, the contents as recited in patents U.S. Pat. Nos. 5,211,741, 5,261,942, 5,580,367 and 5,261,942 are substantially the same as those in U.S. Pat. No. 4,940,490.
The prior art tantalum powder useful for an intermediate voltage capacitor mainly include flaked tantalum powder prepared by reducing tantalum powder with sodium or from tantalum ingot. These tantalum powders commonly have the disadvantages of bad particle shapes, and a too high oxygen content. More importantly, anode block prepared from the tantalum powders has poor electrical properties, for example, bad breakdown resistance, high leakage current, and too high loss or the like, and especially, the disadvantage of the too high leakage current can influence the applications of the tantalum powder.